1. Field of the Invention
The invention relates generally to electrostatic chucks for holding a workpiece and, more specifically, to an electrostatic chuck having a flex circuit with improved puncture resistance and a surface topography that promotes heat transfer from a semiconductor wafer to the chuck.
2. Description of the Background Art
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer process chamber. Electrostatic chucks secure a workpiece by creating an electrostatic attractive force between the workpiece and the chuck. A voltage is applied to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The opposite charges pull the workpiece against the chuck, thereby retaining the workpiece. In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a support during processing. The support may form both an electrode (in electrostatic chuck applications) and a heat sink. These chucks find use in etching, chemical vapor deposition (CVD), ion implantation, and physical vapor deposition (PVD) applications.
The materials and processes used to process a wafer are extremely temperature sensitive. Should these materials be exposed to excessive temperature fluctuations due to poor heat transfer from the wafer during processing, performance of the wafer processing system may be compromised resulting in wafer damage. To optimally transfer heat between the wafer and a chuck, a very large electrostatic force is used in an attempt to cause the greatest amount of wafer surface to physically contact a support surface of the chuck. However, due to surface roughness of both the wafer and the chuck, small interstitial spaces remain between the chuck and wafer that interfere with optimal heat transfer.
To achieve further cooling of the wafer during processing, an inert gas such as Helium is pumped into the interstitial spaces formed between the wafer and the support surface. This gas acts as a thermal transfer medium from the wafer to the chuck that has better heat transfer characteristics than the vacuum it replaces. The chuck generally has a contoured surface to maximize heat transfer and to prevent the heat transfer gas from escaping into the surrounding low pressure atmosphere (i.e., the reaction chamber). To further enhance the cooling process, the chuck is typically water-cooled via conduits within the support. This cooling technique is known as backside gas cooling.
An electrostatic chuck often has a flex circuit comprised of a conductive material, such as copper, encased in a flexible dielectric material such as polyimide. The flex circuit has a sculptured surface that enhances backside gas cooling by providing specific channels or grooves for conveyance of the heat transfer gas across the entire bottom surface of the wafer and into the interstitial spaces. The flex circuit may be shaped in various configurations such as a flat plate across most of the support surface, as a series of concentric rings or radial arms to disperse the heat transfer gas across the entire bottom surface of the wafer. The sculptured surface of the flex circuit includes contours such as ribs, grooves or channels provided therein to further improve heat transfer gas flow uniformity.
A disadvantage of prior art chucks that utilize a laminated flex circuit with a sculptured surface is that dielectric materials, such as polyimide, typically used in existing flex circuits are likely to puncture after repeated clamping of wafers in a wafer processing system. A puncture in the upper dielectric of a flex circuit of an electrostatic chuck can lead to a short circuit between the chuck electrode and the wafer. When the chuck electrode is energized, such a short circuit produces a large current flowing through the wafer that damages the wafer. Such a short circuit can only be repaired by replacement of the chuck causing stoppage of production and a decline in productivity.
Therefore, there is a need in the art for a semiconductor wafer processing chuck having a contoured flex circuit that is resistant to punctures and a concomitant method of manufacturing same.